504 



present in opalescing drops, is changed into a red coloring matter by means 

 of an autumnal abundant formation of acid, a reddening and yellowing of 

 the leaves follow. The leaf turns brown, however, if on the other hand, 

 there predominates the formation of brownish yellow masses observed by G. 

 Kraus^ and Haberlandf- with the destruction of the form of chloroplasts, 

 which masses C. Kraus considered as the products of oxidation and humi- 

 faction of the carbo-hydrates and which, as I believe, can directly arise from 

 the decomposition of the chloroplasts. 



The most frequent, but certainly not the only cause of the red color- 

 ation, is the lowering of the temperature, whereby the action of the light 

 becomes relatively excessive. It is not the absolute values of light and heat 

 which are determinative here, but the relative ones, i. e., those coming under 

 consideration in relation to one another. A lowering of the temperature 

 reduces the process of chlorophyll formation, while it sustains in full activ- 

 ity that of oxidation, which, forming Brenz catechin, requires more light^, 

 and initiates the red coloration. If the activity of the chlorophyll apparatus 

 is increased, i. e., more carbo-hydrates are formed, the accessible oxygen is 

 no longer sufficient for so high a degree of oxidation and the process of red 

 coloration is suppressed. If, however, the work of the chloropyll is arti- 

 ficially retarded by a lack of nutriment and moisture, then the oxygen acces- 

 sible in the cell can suffice to reoxydize to a high degree the material which 

 has become more scanty; in this case the autumn color occurs even in 

 summer. 



As has been mentioned already, 1 observed in August, with girdling 

 experiments on Crataegus, that the autumn coloring occurred even during 

 the intense heat of summer and that at times it was possible with somewhat 

 more solid leaves to bring the tip of the leaf which had been left on the tree 

 to a bright red autumn change of color by breaking the midrib while the leaf 

 base, lying below the sharp point of breaking, retained its normal deep green 

 color. Besides this, in the course of the summer, we find, in many plants, 

 vhat the first formed leaves of the annual growth, which have quickly lived 

 out their life, assume their autumnal coloration in the heat of summer 

 (Ampelopsis). Places on young red leaves, which have been covered, 

 remain greener. We will take up these conditions again under "Defoliation 

 due to frost." The winter preparation of evergreen plants will be taken up 

 thoroughly in the section on "Theories as to the Nature of Frost Action." 



Frosting and Freezing to Death. 



In the literature on this subject, we find different conceptions of the 

 term "freezing to death." Death which gradually sets in in a plant because 

 it has not obtained the warmth necessary for carrying through its normal 

 functions has been explained in part as freezing. On the other hand, only 



1 Okonom. Fortschritte 1872. Nos. 1 and 2. 

 :; Biedermanns Centralbl. 1876, II, p. 48. 



3 Batalin. tlber die Einwirkung des Lichtes auf die Bildung des rnten Pig- 

 mentes. Acta Hort. Fctrop. VI. 



